The present invention relates generally to permanent magnet motor assemblies, and more particularly to devices and methods for improving motor efficiency by reducing parasitic losses.
The ability of flywheels to accept and release energy over relatively short time periods has been known for many years and energy storage flywheels have been used, or proposed for use, in a variety of applications. Such proposed and actual use applications include motor vehicle applications and stand alone supplemental energy sources.
Flywheels store energy by spinning a rotor about an axis, the rotor having magnets attached thereto for generating a magnetic field (in the form of magnetic flux lines), which reacts with a motor stator to produce torque. The motor stator is in the form of a plate with a cut-out section for receiving a copper coil. During acceleration of the rotor, energy is transferred by the motor to the rotor and can be stored in the rotor. However, some of the energy is not transferred to the rotor, but is instead lost in the motor stator, resulting in undesirable heat generation in the stator and reducing the efficiency of the motor. Losses in the motor stator, e.g. losses resulting from eddy currents, are referred to as xe2x80x9cparasiticxe2x80x9d losses. Therefore, it would be desirable to provide an improved flywheel having a mechanism to reduce or substantially eliminate such parasitic losses between the magnet and the stator.
A permanent magnet motor assembly is disclosed including a device and method for shielding a stator of the motor against stray magnetic fields which produce parasitic losses. The motor assembly includes the stator, a rotor, and a shield, wherein the stator has a cut-out section or groove for receiving a coil. The coil is preferably made of strands of copper having a specified number of turns. A rotor positioned adjacent the coil rotates about an axis, as a result of interaction between current in the coil and the magnetic field generated by magnets mounted on the rotor.
Each of the magnets preferably is mounted along a length of the rotor and includes a proximal end positioned near the stator and another end opposite the proximal end. The shield covers the proximal end of each magnet to reduce magnetic flux leakage through the proximal end of the magnet. Magnetic flux leakage occurs when portions of the magnetic field, instead of reaching the rotor, either interfere with the stator or escape the system. Such leakage results in parasitic losses in the motor assembly. Specifically, these stray magnetic field lines can produce eddy current losses in the stator, generating heat and further reducing system efficiency. By providing a shield covering the proximal end of each magnet, such stray magnetic field lines are re-directed away from the stator, thereby improving system efficiency.
Preferably, in the herein described invention, the rotor is made of magnetic steel and the shield is also made of magnetic steel. In one illustrative embodiment, the shield is a magnetic steel cup extending around the proximal end of the magnet which can be designed to fit tightly against the rotor, such that the magnetic steel cup is substantially integral with the rotor. In another illustrative embodiment, the shield is a snap-fit ring of magnetic steel extending around the rotor and fitting within a cut-out portion of the rotor.
Other aspects and embodiments of the invention are discussed below.